This invention relates to passively scanning the gamma radiation emission count of a nuclear fuel contained within a fuel rod to determine enrichment levels and uniformity throughout the fuel rod.
Nuclear energy has become an important source and supply of energy for many countries throughout the world. The nuclear fuel elements used in the generation of energy usually are composed of a multiplicity of nuclear fuel rods encased within a central reactor core. The nuclear fuels used are typically composed of small pellets of uranium or some other fissionable isotope such as plutonium encased within a long tubular housing commonly known as the fuel rod. Even though a number of pellets are contained within each fuel rod, each pellet must have a proper percentage of fissionable isotope (enrichment) as compared with other pellets.
Natural uranium has an enrichment factor of approximately 0.71% which corresponds to the percentage of the highly fissionable uranium-235 (U.sup.235) isotope. After processing to enrich the U.sup.235 isotope content, the uranium content typically has a U.sup.235 enrichment factor ranging from three to five percent and is commonly a uranium dioxide (UO.sub.2) powder derived from gaseous uranium hexaflouride (UF.sub.6). The powder is pelletized for placement within the fuel rods and the fuel rods scanned to assure that all pellets are of a uniform enrichment. The scanning operation is important since a non-homogenous fuel rod having varying enrichments throughout or a deviant average enrichment varying from an established norm could create severe variations in fuel burn-up or heating while a reactor is in operation.
Two methods have been previously employed to scan a nuclear fuel contained within a fuel rod for variations in enrichment along the rod (i.e., enrichment uniformity); either a passive or an active system has been used. A passive system detects the natural radiation of the nuclear fuel while an active system induces an additional radiation in the nuclear fuel above that amount irradiated naturally and detects that additional radioactivity. Of these two systems, the most efficient and commonly used method previously employed has been the active system. In an active system, a primary radiation consisting of neutrons from a source such as Californium-252 bombards the nuclear fuel within a fuel rod inducing a secondary radiation of gamma emissions and prompt or delayed fast neutrons. This secondary radiation is then counted to determine fuel content and enrichment.
Even though these active systems have proven feasible in the past for scanning ordinary fuel rods containing nuclear fuels, difficulties often arise when scanning recently manufactured nuclear fuels. Manufacturers have begun to add thermal neutron absorbing materials (burnable poisons) to the nuclear fuels encased within a fuel rod which makes scanning such a rod by the common active system impracticable since an ambiguity exists between the primary radiation absorption of the burnable poison and the effects of the fuel enrichment. These burnable poisons such as Gadolinium, Europium and Boron are added to the nuclear fuel to reduce reactivity variations during the fuel burn-up in nuclear reactors. The loss of reactivity through the depletion of the nuclear fuel is partially compensated through the destruction of the burnable poison by neutron absorption during reactor operation.
Since the more commonly used active system is limited in its accuracy by the burnable poison content within a nuclear fuel, a passive system is the other alternative which can be used for scanning a nuclear fuel having with it associated burnable poisons. A passive system detects the radiation caused by the natural decay of the nuclear fuel. For example, the 185 KeV (thousand electron volts) gamma radiation emitted by U.sup.235 may be detected in order to determine the enrichment factor of a nuclear fuel using U.sup.235 as the fissionable isotope. The amount of burnable poison associated with the nuclear fuel has no bearing upon the natural gamma radiation emitted by the fuel. Unfortunately, prior attempts to use passive systems for determining enrichment values have been limited since a gamma radiation detector or scintillator of the type commonly used is not able to reliably detect the very low natural irradiations which are associated with conventional fuel rods, and thus the error rate was relatively high. Not only were the prior passive systems limited by this relatively high error rate, but other limitations hindered the passive system such as interference from other gamma radiation energies. Many detectors have an unacceptable poor resolution making the differentiation between the various spectral energy lines difficult. For example, in a nuclear fuel with a high U.sup.238 content, the higher energy gamma radiation from the U.sup.238 daughter, Protactinium-234 is great enough to interfere with the resolution of a detector.
Therefore, it is an object of the present invention to provide a method of and an apparatus for passively scanning the gamma radiation emission count of a nuclear fuel contained within a fuel rod to determine enrichment uniformity, and which overcomes the abovenoted limitations of the prior passive systems.
It is a further object of the present invention to provide a method of and apparatus for passively scanning the gamma radiation emission count of a nuclear fuel contained within a fuel rod to determine enrichment uniformity, and which are fully automatic.